The newer 27" 'Thin Series' iMac's have two drive interfaces: A SATA III for a traditional spinning disk HD and a custom PCIe blade drive which is similar to the M.2 drives but uses a custom physical interface. M.2 SSD's are not compatible in this system!
A Fusion Drive'd system is were these two drives are bonded together logically into a single drive. Thus leveraging the benefits of each, a traditional HD for its deep storage and the SSD for is speed.
Here's the IFIXIT guide to replace (install) the PCIe x4 SSD in this system: iMac Intel 27" Retina 5K Display SSD Replacement as you can see we've put a lot of warnings as this is a very big job and if you make a mistake quite costly! So I would not recommend trying to do this if you can avoid it.
Here is the IFIXIT guide to replace (install) a SATA drive: iMac Intel 27" Retina 5K Display Hard Drive Replacement as you can see its a lot less work but if you aren't careful you can damage the display assembly! So make sure you use the correct tools and get new adhesive strips (maybe a few sets just in case).
If you decide to go with a Fusion Drive config remember you need to break the fusion set before swapping out the HD and you can't rebuild a Fusion Drive using two SSD's.
So, the bottomline here is buy a iMac with the biggest SSD you can afford (you can custom order a SSD only system), then if you must upgrade later on focus on just replacing the HD and you can only use a traditional HD in a Fusion Drive setup. You can still run the two drives independently if you want to! So you can run dual SSD's just not in a fusion drive setup.